Sorry, should have clarified.
By Rooting, it means that you can gain full access/control of the underlying Android operating system. Performing the documented hack, lets you re-insert the modified internal (not removable unless you significantly disassemble the Nook) Micro SD where the Operating System is stored (as well as the user accessible portion of the file system where the My documents, My screensaver, My wallpaper, My music folders reside). You can then run a Android tool on another computer on your network and access your nook (you have to figure out what IP address your router gave it when it associated to your WiFi router's wireless network) via a command shell. From there you have the full run of the operating system, you can view/modify files, apparently you can run applications on your nook as well as see what applications are running.
Not for the faint of heart. But definitely the keys to the whole OS in terms of hacking it.